Interconnected hollow nanostructures containing high capacity active materials for use in rechargeable batteries

ABSTRACT

Provided are electrode layers for use in rechargeable batteries, such as lithium ion batteries, and related fabrication techniques. These electrode layers have interconnected hollow nanostructures that contain high capacity electrochemically active materials, such as silicon, tin, and germanium. In certain embodiments, a fabrication technique involves forming a nanoscale coating around multiple template structures and at least partially removing and/or shrinking these structures to form hollow cavities. These cavities provide space for the active materials of the nanostructures to swell into during battery cycling. This design helps to reduce the risk of pulverization and to maintain electrical contacts among the nanostructures. It also provides a very high surface area available ionic communication with the electrolyte. The nanostructures have nanoscale shells but may be substantially larger in other dimensions. Nanostructures can be interconnected during forming the nanoscale coating, when the coating formed around two nearby template structures overlap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 13/891,035, filed May 9, 2013, titled“Interconnected Hollow Nanostructures Containing High Capacity ActiveMaterials for Use in Rechargeable Batteries,” which is a divisional ofU.S. patent application Ser. No. 12/787,138 (now U.S. Pat. No.8,450,012), filed May 25, 2010, titled “Interconnected HollowNanostructures Containing High Capacity Active Materials for Use inRechargeable Batteries,” which claims the benefit of U.S. ProvisionalPatent Application No. 61/181,637, filed May 27, 2009, titled“Core-Shell High Capacity Nanowires for Battery Electrodes,” and alsoclaims the benefit of U.S. Provisional Patent Application No.61/183,529, filed Jun. 2, 2009, entitled “Electrospinning to FabricateBattery Electrodes.” Each of these prior applications is incorporatedherein by reference in its entirety and for all purposes.

BACKGROUND

High capacity electrochemically active materials are desirable forbattery applications. However, these materials exhibit substantialvolume changes during battery cycling, e.g. swelling during lithiationand contraction during delithiation. For example, silicon swells as muchas 400% during lithiation to its theoretical capacity of about 4200mAh/g or Li_(4.4)Si. Volume changes of this magnitude causepulverization of active materials structures, losses of electricalconnections, and capacity fading.

Forming high capacity active materials into certain types ofnanostructures can address some of these issues. Nanostructures have atleast one nanoscale dimension, and swelling-contracting along thisnano-dimension tends to be less destructive than along large dimensions.As such, nanostructures can remain substantially intact during batterycycling. However, integrating multiple nanostructures into a batteryelectrode layer that has adequate active material loadings is difficult.Such integration involves establishing and maintaining electricalinterconnections and mechanical support over many cycles.

SUMMARY

Provided are electrode layers for use in rechargeable batteries, such aslithium ion batteries, and related fabrication techniques. Theseelectrode layers have interconnected hollow nanostructures that containhigh capacity electrochemically active materials, such as silicon, tin,and germanium. In certain embodiments, a fabrication technique involvesforming a nanoscale coating around multiple template structures and atleast partially removing and/or shrinking these structures to formhollow cavities. These cavities provide space for the active materialsof the nanostructures to swell into during battery cycling. This designhelps to reduce the risk of pulverization and to maintain electricalcontacts among the nanostructures. It also provides a very high surfacearea available ionic communication with the electrolyte. Thenanostructures have nanoscale shells but may be substantially larger inother dimensions. Nanostructures can be interconnected during formingthe nanoscale coating, when the coating formed around two nearbytemplate structures overlap. In certain embodiments, the electrode layeralso includes a conductive substrate, which may be also interconnectedwith the nanostructures.

In certain embodiments, a method for preparing an electrode layer ofinterconnected hollow nanostructures is provided. The nanostructuresinclude a high capacity electrochemically active material. The methodmay involve receiving a template that includes template structures,forming a nanoscale template coating of the high capacity materialaround the template structures, and, at least partially, removing and/orshrinking the template to form an electrode layer. The template may befabricating using an electrospinning technique, e.g., electrospinning apolymeric material to form template nanofibers having a length of atleast about 5 micrometers. The template may include poly-acrylicnitrides (PAN), nylons, polyethylenes, polyethylene oxides, polyethyleneterephthalates, polystyrenes, and/or polyvinyls. In certain embodiments,a template forms a layer having a thickness of between about 10micrometer and 150 micrometers and a porosity of between about 20% and80%.

In certain embodiments, a method involves pre-treating a template bycompressing the template, thermally stabilizing the template, and/orcarbonizing the template. For example, a template may be thermallystabilizes by heating the template in an argon atmosphere to betweenabout 150° C. and 250° C. for at least about 2 hours. In certainembodiments, a method involves forming a nanoscale substrate coating ofthe high capacity materials over a conductive substrate surface adjacentto the template. The nanoscale template coating and the nanoscalesubstrate coating may partially overlap to interconnect the conductivesubstrate and the interconnected hollow nanostructures.

In certain embodiments, forming a nanoscale template coating around thetemplate involves two deposition stages, e.g., an initial depositionstage and a bulk deposition stage. An initial deposition stage may beperformed at initial process conditions such that no substantial shapedistortions of the template occur during the initial deposition stage. Abulk deposition stage performed at bulk process conditions that aredifferent from the initial process conditions. The bulk conditions mayprovide a higher deposition rate of the nanoscale template coatingduring the bulk deposition stage.

In certain embodiments, at least some removal or shrinking of a templateoccurs during formation the nanoscale template coating around thetemplate structures. Partially removal or shrinking of a template mayinclude one or more of the following operations: burning the template ata temperature of at least about 300° C. in presence of an oxidant,chemical etching the template, and annealing the template.

In certain embodiments, a method may involve forming a second coatingover the interconnected hollow nanostructures. The second coating may beconfigured to increase an electronic conductivity of the electrodelayer, improve solid electrolyte interphase (SEI) characteristics of theelectrode layer, and/or to limit structural changes of theinterconnected hollow nanostructures.

In certain embodiments, an electrode layer for use in a rechargeablebattery includes interconnected hollow nanostructures having an aspectratio of at least about four, a length of at least about 5 micrometers,and a shell thickness of less than about 100 nanometers. Thesenanostructures may include one or more high capacity electrochemicallyactive materials. The interconnected hollow nanostructures form internalcavities that provide free space for the high capacity active materialto swell into during cycling of the rechargeable battery. The internalcavities may be substantially inaccessible to an electrolyte of therechargeable battery. In certain embodiments, the electrode layer isconfigured to provide a stable energy capacity of at least about 2000mAh/g after at least 100 cycles based on the weight of the activematerial. An electrode layer may also include junction structuresinterconnecting two or more hollow nanostructures. The junctionstructures may include one or more of the following materials: a highcapacity electrochemically active material, metal, and a polymericbinder.

In certain embodiments, an electrode layer includes a conductivesubstrate. Additional junction structures may interconnect some of theinterconnected hollow nanostructures and conductive substrate. Incertain embodiments, the active material includes silicon, tin, and/orgermanium. An electrode layer may also include an outer layersubstantially covering an outer surface of the interconnected hollownanostructures. The outer layer may include carbon, titanium, silicon,aluminum, and/or copper. In certain embodiments, an electrode layer hasa porosity of between about 20% and 80%. In certain embodiments, arechargeable battery is a lithium ion battery and the active material ofthe electrode layer is a negative active material.

In certain embodiments, a lithium ion battery includes an electrodelayer that contains interconnected hollow nanostructures having anaspect ratio of at least about four, a length of at least about 5micrometers, and a shell thickness of less than about 100 nanometers.The hollow nanostructures may include one or more high capacityelectrochemically active materials. The hollow nanostructures forminternal cavities that provide free space for the high capacity activematerial to swell into during cycling of the rechargeable battery.

These and other aspects of the invention are described further belowwith reference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flowchart of a technique for preparing an electrodelayer that has interconnected hollow nanostructures containing a highcapacity electrochemically active material in accordance with certainembodiments.

FIG. 2A illustrates a perspective schematic view of an assembly thatincludes a substrate and a plurality of template patches in accordancewith certain embodiments.

FIG. 2B illustrates a side schematic view of an assembly that includes asubstrate and a plurality of template structures in accordance withcertain embodiments.

FIG. 3A is a schematic representation of a template having two templatestructures proximate to each other and to the substrate prior to forminga nanoscale coating containing a high capacity material in accordancewith certain embodiments.

FIG. 3B is a schematic representation of an electrode layer containinginterconnected hollow nanostructures formed around the two templates andon the substrate surface that has two joint structures in accordancewith certain embodiments.

FIG. 4 is a schematic representation of a structure at three differentillustrative stages of the electrode layer fabrication process inaccordance with certain embodiments.

FIGS. 5A-5B are top and side schematic views of an illustrativeelectrode arrangement in accordance with certain embodiments.

FIGS. 6A-6B are top and perspective schematic views of an illustrativeround wound cell in accordance with certain embodiments.

FIG. 7 is a top schematic view of an illustrative prismatic wound cellin accordance with certain embodiments.

FIGS. 8A-8B are top and perspective schematic views of an illustrativestack of electrodes and separator sheets in accordance with certainembodiments.

FIG. 9 is a schematic cross-section view of an example of a wound cellin accordance with embodiments.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail to not unnecessarily obscure the present invention.While the invention will be described in conjunction with the specificembodiments, it will be understood that it is not intended to limit theinvention to the embodiments.

INTRODUCTION

High capacity electrochemically active materials can be formed intonanostructures for use in rechargeable batteries. Nanostructures tend todeteriorate much more slowly than larger structures during batterycycling. This is because they pulverize less readily and, therefore,maintain electrical contact the electronically conductive currentcollects of an electrode. Yet, producing an electrode active layercontaining nanostructures with adequate active material loading ischallenging. For example, it is difficult to mechanically arrange,support, and electrically interconnect a multiplicity of nanostructuresand then maintain these arrangements and interconnections over a largenumber of cycles. For example, solid nanoparticles that are only0.05-0.10 micrometers in diameter must rely on many electricalconnections and intermediate structures for electrical conductivity tothe substrate in a typical 50-100 micrometer thick active layer. Initialelectrical connections formed, for example, by direct contacts betweenthe nanoparticles and conductive additives are often lost when thenanoparticles swell during lithiation/charging. This swelling pushesapart the nanoparticles and other components. When nanoparticles shrinkduring delithiation/discharging, many initial connections may be lostleading to “unconnected” active particle, which effectively become“inactive”. Another type of nanostructures is a nanofilm. Nanofilms aretypically less than 0.1 to 0.25 micrometers thick and, therefore, caninclude only very small amounts of the active material per unit area ofelectrode face and therefore have inadequate capacity for most batteryapplications.

Interconnected hollow nanostructures as disclosed herein combine somefavorable characteristics of the nanostructures described above andprovide additional new characteristics and capabilities that werepreviously not attainable by solid structures. One example of hollowsilicon nanostructures for use in battery electrodes are shown in FIG.3B. These hollow nanostructures have nanoscale shells, while otherdimensions could be substantially larger. For example, nanotubes canhave an inner diameter of up to 5 micrometers and a length of up to 50micrometers and sometimes even longer. These relative large nanotubescan still withstand lithiation/delithiation cycling without substantialpulverization because of their thin nanoscale shells. Such nanotubeshave inside cavities available for accommodating expanding activematerials. Without being restricted to any particular theory, it isbelieved that high capacity materials, such as amorphous silicon,exhibit different swelling/contraction behavior when arranged intohollow structures relative to other nanostructures. This differenceproved to be unexpectedly beneficial to the stability of the novelnanostructures during battery cycling, which is evident from theexceptional cycle life performance of the batteries assembled with thenovel electrode layers. For example, the experiment results demonstratedover 2000 mAh/g stable capacity after more than 140 cycles.

Interconnected hollow nanostructures can be prepared by forming acoating containing a high capacity material around template structures.In certain embodiments, when template structures are positionedrelatively close to each other or to the substrate, the coating layersoverlap forming joint structures. The template is then partially orcompletely removed and/or shrunk to form hollow cavities inside thenanostructures. These and other aspects of electrode layers andfabrication techniques will now be described in more detail.

Process

FIG. 1 is a process flowchart depicting a technique for preparing anelectrode layer that has interconnected hollow nanostructures containinga high capacity electrochemically active material in accordance withcertain embodiments. The process 100 may start with receiving a templatecontaining multiple template structures in an operation 102. Examples oftemplate structures include high-aspect ratio (e.g., at least about 4,10, or 50) fibers and hollow tubes (e.g., carbon nanotubes), particles(e.g., substantially round particles), flakes, particles or spheres, andother types of template structures. Generally, a template can have anyshape that provides an adequate surface for forming a high capacitymaterial coating that can later be formed into hollow nanostructures. Aprincipal cross-section dimension of the template structures (e.g., adiameter for fibers and particles) may be between about 1 nanometer and5,000 nanometers or, more particularly, between about 10 nanometers and1,000 nanometers or even more particularly between 100 nanometers and500 nanometers. In particular embodiments, the template includesmultiple randomly oriented fibers. The fibers may be straight (e.g.,carbon nanotubes, carbon fibers) or curvy (e.g., electrospun polymerfibers). The fibers may be at least about 100 nanometers long on averageor, more particularly, at least about 1 micrometer long or even at leastabout 50 micrometers.

Individual fibers or other template structures may be positionedrelatively close to each other to ensure that when coating layers aredeposited onto the structures, at least some of the layers overlap andform joint structures. In other words, at least some distances betweentemplate structures should be less than two thicknesses of the coatinglayer. Some template structures may be in a direct contact with eachother. For example, a layer of randomly oriented fibers may be used as atemplate where each fiber will have at least two contact points withother fibers.

In certain embodiments, a template has a porosity of between about 20%and 80% or, more particularly, between about 50% and 70%. Higherporosity may be needed for depositing thicker layers and/or to formelectrode layers that themselves will have high porosities (e.g., toimprove electrolyte migration). The template porosity should bedistinguished from the electrode layer porosity, which depends on otherfactors as well. In certain embodiments, template structures areattached to another common element, e.g., a substrate, such as a currentcollecting substrate. In these embodiments, the template structures mayexist without contacting each other. For example, metal silicidenanowires can be formed on a metal substrate and used as a template.Examples of such templates are described in U.S. Provisional PatentApplication No. 61/310,183, entitled “Electrochemically ActiveStructures Containing Silicides,” filed on Mar. 3, 2010, which isincorporated by reference herein. It should be noted that such templatesmay still have some incidental contacts among template structures.

A template should withstand, at least initially, the process conditionsused during formation of the high capacity coating. While somecollapsing, shrinking, and/or other shape distortions may be allowed,the template should be capable of providing initial mechanical supportfor the formed coating. Any major shape distortions that wouldeventually prevent formation of hollow nanostructures should be avoided.This can be achieved by selecting robust template materials (e.g., heatresistant material) or controlling the coating conditions in specificways, which is described below in more details.

Examples of template materials include various polymer materials (e.g.,poly-acrylic nitrides (PAN) (as in oxidized PAN fibers), nylons,polyethylenes, polyethylene oxides, polyethylene terephthalates,polystyrenes, and polyvinyls), carbon-based materials (e.g., graphite,coke, carbon nanotubes, carbon fibers), metals (e.g., copper, nickel,aluminum), metal oxides, and metal silicides. Template materials thatcan be provided as electrospun fibers or any other forms are describedin U.S. Provisional Patent Application No. 61/183,529, entitled“Electrospinning to Fabricate Battery Electrodes,” filed Jun. 2, 2009,which is incorporated herein by reference.

A template may form a layer that has a thickness of between about 1micrometer and 300 micrometers or, more particularly, between about 10micrometers and 150 micrometers, or even between about 50 micrometersand 100 micrometers. This layer may define future boundaries of theelectrode layer. A template layer may be formed as a discrete patch thatdefines dimensions (e.g., length and/or width) of the future electrode.This example is further described below in the context of FIG. 2A.

In certain embodiments, a template layer is positioned adjacent to asubstrate layer prior to coating. For example, a substrate layer may bea thin foil having a thickness of between about 5 micrometers and 50micrometers or, more particularly, between about 10 micrometers and 30micrometers. In other embodiments, a substrate layer is a mesh,perforated sheet, foam, and the like. In these embodiments, the templatemay be positioned within the substrate layer, e.g., template structuresare dendrites extending from the mesh wires. Examples of substratematerials include copper, coated and un-coated metal oxides, stainlesssteel, titanium, aluminum, nickel, chromium, tungsten, metal nitrides,metal carbides, carbon, carbon fiber, graphite, graphene, carbon mesh,conductive polymers, or combinations of the above including multi-layerstructures. In certain embodiments, a substrate may have functionaland/or protective layers, e.g., a catalyst layer, diffusion barrierlayer, and/or adhesion layer. Various examples of such layers aredescribed in U.S. Provisional Patent Application No. 61/260,297,entitled “Intermediate Layers for Electrode Fabrication,” filed on Nov.11, 2009, which is incorporated herein by reference.

FIG. 2A illustrates a perspective schematic view of an assembly 200 thatincludes a substrate 202 and a plurality of template patches 204 inaccordance with certain embodiments. This assembly 200 may be formed,for example, by electrospinning template fibers directly onto thesubstrate 202 or depositing previously electrospun fibers onto thesubstrate 202. In some cases, the template patches are provided on bothsides of the substrate. Various examples of active layer arrangementsare described below in the context of an electrode and a battery. Incertain embodiments, a template patch 204 has a length corresponding toa length or width of the final battery electrode. The support substrate202 may be cut, at a later stage in the fabrication, in order toseparate the template patches into individual electrodes.

FIG. 2B is a side schematic view of an assembly 210 that includes asubstrate 212 and a plurality of template structures 214 in accordancewith certain embodiments. Some of the template structures 214 can besubstrate rooted, which can provide a mechanical connection between thetemplate and the substrate 212. If a portion of the template laterremains in the electrode layer, this substrate rooting may help toprovide at least some mechanical support and/or electronic conductivityto the electrode layer. Substrate rooting can take many forms asdiscussed in U.S. patent application Ser. No. 12/437,529, entitled“Electrode Including Nanostructures for Rechargeable Cells”, filed onMay 7, 2009, which is incorporated herein by reference. In someembodiments, the template is not bonded to the substrate. Forming anactive material coating may provide bonding to the substrate asexplained below in the context of FIGS. 3A and 3B. Other bondingoperations and/or materials may be used. For example, an adhesion layercan be formed on a substrate to enhance adhesion of active materialstructures to the substrate. In other embodiments, the templatestructures 214 are temporarily supported on the substrate 212 with, forexample, a binder, which is later removed prior or during formation ofthe high capacity material coating. In other examples, the templatestructures 214 are supported on the substrate 212 using electrostatic ormagnetic forces.

Returning to FIG. 1, the received template may be pre-treated in anoperation 104 prior to formation of the high capacity coating. Examplesof pre-treatment operations include treatments that interconnecttemplate structures, attach the template to a substrate, achieve certaindesired shapes and/or porosity characteristics of the template,thermally stabilize the template, modify composition of the template(e.g., carbonize polymers) and electrical conductivity, and accomplishvarious other purposes. In certain embodiments, a template is compressedto reduce its porosity or achieve a target thickness, e.g., in order toprovide a higher level of interconnection among resulting hollownanostructures. A polymer-based template may be heated to at least about100° C. or, more particularly, to at least about 150° C. during thecompression.

In the same or other embodiments, a polymer-based template may bethermally stabilized in operation 104. For example, polymer basedtemplate structures can be heated to between about 100° C. and 300° C.or, more particularly, to between about 150° C. and 250° C. for a periodof between about 1 hours and 48 hours or, more particularly, betweenabout 12 hours and 28 hours, in an inert atmosphere (e.g., argon). Incertain embodiments, an oxidizing agent may be added into the chamber toform an oxide layer (e.g., for form oxidized PAN) and even partiallyburn out template structures to change their dimensions and/orcomposition.

A polymer-based template may also be partially or completely pyrolizedor carbonized. For example, carbonization may help to improve thermalstability and surface properties for subsequent processing. In certainembodiments, carbonization involves heating the template to betweenabout 300° C. and 2000° C. or, more particularly, to between about 500°C. and 1700° C., for a period between about 0.25 hours and 4 hours or,more particularly, to between about 0.5 hours and 2 hours. Carbonizationor pyrolysis may be performed in various environments, which aretypically inert or reducing.

The process 100 may continue with forming a nanoscale coating containingone or more high capacity active materials around the template in anoperation 106. The coating may be, on average, between about 5nanometers and 1000 nanometers thick or, more particularly, betweenabout 10 nanometers and 500 nanometers thick or even between about 20nanometers and 100 nanometers. This thickness may be determined, atleast in part, by the composition of the coated materials and thecross-section dimension (e.g., diameter) of the shell formed by thecoated materials and determined by the template dimensions, which aredescribed above.

High capacity active materials are defined as any electrochemicallyactive materials that have a theoretical lithiation capacity of at leastabout 600 mAh/g. Examples of such materials include silicon containingmaterials (e.g., crystalline silicon, amorphous silicon, othersilicides, silicon oxides, sub-oxides, oxy-nitrides), tin-containingmaterials (e.g., tin, tin oxide), germanium, carbon-containingmaterials, metal hydrides (e.g., MgH₂), silicides, phosphides, andnitrides. Other examples include carbon-silicon combinations (e.g.,carbon-coated silicon, silicon-coated carbon, carbon doped with silicon,silicon doped with carbon, and alloys including carbon and silicon),carbon-germanium combinations (e.g., carbon-coated germanium,germanium-coated carbon, carbon doped with germanium, and germaniumdoped with carbon), and carbon-tin combinations (e.g., carbon-coatedtin, tin-coated carbon, carbon doped with tin, and tin doped withcarbon). In certain embodiments, a coating may include active materialsthat do not reach the theoretical lithiation capacity listed above.These materials can be used in combination with the high capacity activematerials or by themselves.

This technique may be used to form both negative and positive electrodelayers. Examples of positive electrochemically active materials includevarious lithium metal oxides (e.g., LiCoO₂, LiFePO₄, LiMnO₂, LiNiO₂,LiMn₂O₄, LiCoPO₄, LiNi_(1/3)Co_(1/3)Mn_(1/3) 0 ₂,LiNi_(X)Co_(Y)Al_(Z)O2, LiFe₂(SO4)₃), carbon fluoride, metal fluoridessuch as iron fluoride (FeF₃), metal oxide, sulfur, and combinationthereof. Doped and non-stoichiometric variations of these positive andnegative active materials may be used as well. Examples of dopantsincludes elements from the groups III and V of the periodic table (e.g.,boron, aluminum, gallium, indium, thallium, phosphorous, arsenic,antimony, and bismuth) as well as other appropriate dopants (e.g.,sulfur and selenium).

In certain embodiments, the coating forms multiple joint structureswhere at least two coating layers covering nearby template structuresoverlap. Furthermore, the coating may also be deposited on a substrate,if one is present near the template. In this case, join structures canalso form at the substrate interface. In other words, the coating can bedescribed as an interconnected network of multiple layers formed aroundthe template structures and, in certain embodiments, a layer formed onthe substrate surface. The layers formed around the templates in turnform shells or parts of the shells of the resulting interconnectedhollow nanostructures. A particular example of this process andcorresponding structures will now be explained in more detail in thecontext of FIGS. 3A and 3B.

FIG. 3A illustrates an initial assembly 301 that includes a substrate302 and two template structures 304 a and 304 b prior to formation ofthe active material coating around the structures. In certainembodiments, this assembly 301 is provided as a template. The structures304 a and 304 b are shown to have variable spacing between thestructures and between the bottom structure 304 a and the substrate 302.In some areas this spacing is less than two thicknesses of the highcapacity coating, which will result in formation of joint structures inthese locations.

FIG. 3B illustrates a processed assembly 303 after forming the highcapacity coating that includes three layers 306 a, 306 b, and 306 c. Theoriginal template structures 304 a and 304 b may or may not be presentin this assembly 303. The coating layers are shown to form two jointstructures 308 and 310. Specifically, the joint structure 308 is formedwhen the layers 306 a and 306 b overlap. As indicated above, thetemplate structures 304 a and 304 b were spaced apart by less than twocoating thicknesses at this location. Likewise, the joint structure 310was formed when layers 306 a and 306 c overlap caused by the spacingbetween the bottom template structure 304 a and the substrate 302. Thesejoint structures 308 and 310 can provide both mechanical support for theresulting hollow nanostructures and electronic pathways between thelithiation sites and the substrate 302.

In certain embodiments (not shown), a template may include at least twodifferent types of template structures. One type may have large surfaceareas available for coating, i.e., surface template structures. Thesesurface template structures can be larger, have higher aspect ratios,and have predominant presence in the template in order to provide enoughsurface area. Another type of structures may be primarily used toincrease a number of joint structures formed in the electrode layer,i.e., interconnecting template structures. Interconnecting templatestructures may be generally smaller and have low aspect ratios in orderto fit between the surface template structures. For example, nanofibersmay be used in combination with nanoparticles as a composite template toachieve this result.

High capacity coating may be formed using various deposition techniques,such as chemical vapor deposition (CVD), which includes plasma enhancedCVD (PECVD) and thermal CVD, electroplating, electroless plating, and/orsolution deposition techniques. PECVD technique examples will now bedescribed in more details. A template is heated to between about 200° C.and 400° C. or, more specifically, to between about 250° C. and 350° C.A process gas with a silicon containing precursor (e.g., silane) and oneor more carrier gases (e.g., argon, nitrogen, helium, and carbondioxide) is introduced into the process chamber. In a specific example,a combination of silane and helium is used with a silane concentrationof between about 5% and 20%, or more particularly between about 8% and15%. The process gas may also include a dopant containing material, suchas phosphine. The chamber pressure may be maintained at between about0.1 Torr to 10 Torr or, more specifically, at between about 0.5 Torr and2 Torr. To enhance silane decomposition, plasma may be ignited in thechamber. Using plasma may help lowering the template temperature, whichmay be important to preserve template integrity. In certain embodiments,a pulsed PECVD method is employed.

The following process (i.e., RF power and flow rates) parameters areprovided for a STS MESC Multiplex CVD system available from SurfaceTechnology Systems in Newport, United Kingdom. This system can processtemplates/substrates that are up to four inches in diameter. It shouldbe understood by one having ordinary skills in the art that theseprocess parameters can be scaled up or down for other types chambers andsubstrate sizes. The RF power may be maintained at between about 10 Wand 100 W and the overall process gas flow rate may be kept at betweenabout 200 sccm and 1000 sccm or, more particularly, at between about 400sccm and 700 sccm.

In a specific embodiment, forming an amorphous silicon coating isperformed in a process chamber maintained at a pressure of about 1 Torr.The process gas contains about 50 sccm of silane and about 500 sccm ofhelium. In order to dope the active material, about 50 sccm of 15%phosphine may be added to the process gas. The substrate is kept atabout 300° C. The RF power level is set to about 50 Watts.

As mentioned above, it is sometimes needed to control processconditions, at least initially, during operation 106 in order to avoidmajor shape distortions of template structures, especially polymer-basedtemplate structures. At some point during the coating formationoperating, an initially formed coating can be relied on for furthermechanical support, and the process conditions can be adjusted withoutfurther concerns about the shape distortions. For example, the processconditions may be adjusted to increase the deposition rate of thecoating.

In certain embodiments, operation 106 involves two or more stagesperformed using different process conditions, e.g., an initialdeposition stage and a bulk deposition stage, also termed a first stageand a second stage. In one example, forming the high capacity coatingbegins at a lower temperature and, at some point during operation 106,the temperature is increased. The changes in the process conditions maybe gradual or stepwise. The changes may be implemented when at leastabout 10% of the coating is formed or, more particularly, when at leastabout 25% or even when at least about 50% of the coating layer is formed(e.g., based on the coating thickness or weight). In certainembodiments, the initial stage is performed when the template is heatedto less than about 250° C. or, more particularly, to less than about200° C. or even to less than about 150° C. The temperature may be thenincreased to at least about 150° C. or, more particularly, to at leastabout 200° C. or even to at least about 250° C. The initial depositionstage will generally be performed under conditions that maintain thephysical integrity of the template. In such embodiments, thoseconditions are maintained until the coating becomes self-supporting.After that point, more aggressive conditions may be employed to, e.g.,increase the deposition rate. These more aggressive conditions in thebulk deposition stage can be employed without regard to the physicalintegrity of the template. Note that the template is largely sacrificialin many embodiments. After it serves the role of defining a form for theactive material, it becomes expendable.

FIG. 4 is a schematic representation of a structure at threeillustrative stages of the electrode layer fabrication process inaccordance with certain embodiments, which show an example of thetemplate deformation, more specifically partial removal of the template,during the coating operation. It should be noted that in general thetemplate can be partially or completely removed and/or shrunk during thecoating formation. Alternatively, the template may remain completelyintact during this operation. In these later embodiments, the templateis then at least partially or completely removed and/or shrunk in lateroperations. It should be noted that in addition or instead of changingthe template shape, the template may also change its composition duringoperation 106, e.g., carbonize, or during later operations.

The first stage 401 shows an uncoated template structure 402. This stagemay exist prior to initiating operation 106. The template structure 402may be solid as shown in FIG. 4 or hollow (not shown). At the secondillustrative stage 403, the same template structure 402 is shown with athin coating 404 of the high capacity material formed around thestructure 402. As mentioned above, the process conditions may beselected such that no substantial shape deformation to the templatestructure 402 appears at this stage. At this stage 403, the initiallyformed coating 404 may be sufficiently strong to support itself with orwithout any help from the structure 402. The process conditions,therefore, can be changed at this point. The new process conditions maycause substrate removal and/or shrinkage of the template, which resultsin a cavity 410 formed within the remaining template structure 408 asshown in the third illustrative stage 405. Most of the coating may beformed in between the second and third stages resulting in the finalcoating 406.

Returning to FIG. 1, the process 100 may continue with partial orcomplete removal and/or shrinkage of the template material from theelectrode layer in operation 108. This operation is optional. As notedabove, removal and/or shrinking may also occur during the coatingformation in operation 106. In certain embodiments, no additionalremoval and/or shrinkage are needed. In other embodiments, removaland/or shrinkage occur during operation 106 and again in operation 108.In yet other embodiments, removal and/or shrinkage are performed onlyduring operation 108 and the template stays intact during operation 106.For example, a metallic template used to form an amorphous silicon layermay be unaffected by the layer formation and can be removed later byetching.

A removal/shrinking technique during operation 108 depends, in part, ontemplate and coating materials. For example, polymeric templates can beburned out or annealed at high temperature, e.g., by heating to at leastabout 300° C. or, more particularly, to at least about 400° C. or evento at least about 500° C. It may take at least about 1 hour or, moreparticularly, at least about 4 hours to burn out sufficient amount ofthe template materials. In certain embodiments, a template (e.g., ametallic template) is removed by chemical etching. For example, asilicon oxide core template may be etched by a hydrofluoric (HF) acidsolution. In general, acid solutions can be used to etch away variousmetal or silicide fibers used as initial templates.

In certain embodiments, substantially all template material is removedfrom the nanotubular structure. In other embodiments, a portion of thetemplate material remains. It should be noted that this remainingmaterial may have a different composition than initial templatematerials (e.g., a polymer may be converted into carbon, etc.). In thesame or other embodiments, residual template materials or theirderivatives make up less than about 25% or, more particularly, less thanabout 10% by weight of the electrode layer. These residual materials maybe used to enhance electronic conductivity and electrochemical capacityto the electrode layer, protect from undesirable SEI layer formations,limited the structural change (such as volume expansion), and otherpurposes.

The process 100 may continue with forming a second coating over theinterconnected hollow nanostructures in operation 110. A second coatingmay be formed on the outer surfaces of the nanostructures and thereforereferred to as an outer coating. In general, the second coating mayinclude carbon, metals (e.g., copper, titanium), metal oxides (e.g.,titanium oxide, silicon oxide), and metal nitrides, and metal carbides.In certain embodiments, a carbon content of the outer coating is atleast about 50% or, in more specific embodiments, at least about 90% orat least about 99%. In certain specific embodiments, the outer coatingmay include graphite, graphene, graphene oxide, metal oxide (e.g.,titanium oxide), and or other materials. The outer coating may coatsubstantially all exposed surface (e.g., all outside surfaces or bothinside and outside surfaces) of the interconnected hollownanostructures. In certain embodiments, the thickness of the outercoating is between about 1 nanometer and 100 nanometers or, in morespecific embodiments between about 2 nanometers and 50 nanometers.Specifically, the outer coating could be used to prevent a directcontact of the high capacity coating with solvents of the electrolyte(and forming a detrimental SEI layer) yet to allow electro-active ionsto pass to and from the high capacity coating.

A technique for forming an outer coating may involve depositing carbonbased small molecules (e.g., sugars) or polymers followed by annealing.Another technique may involve carbon-based gas pyrolysis (e.g., usingacetylene). For example, a carbon containing outer coating may be formedusing methane, ethane, or any other suitable carbon containingprecursors passed over nickel, chromium, molybdenum, or any othersuitable catalysts (or no catalyst at all) and deposit a carbon layerover the nanostructures. Other methods include coating thenanostructures with organic media, which are later baked leaving carbonresidue. For example, interconnected nanostructures may be dipped into aglucose or polymer solution. After allowing the solution to penetrateinto the nanowire mesh, it is removed from the solution and baked.Glucose leaves carbon residues on the nanostructures. Outer coatingscontaining oxides, such as titanium oxide, may start with depositing abase material (e.g., titanium) from a solution, using an atomic layerdeposition (ALD), or metal plating. Oxides of the base materials arethen formed, for example, by exposing the deposit to oxidants atelevated temperature. Silicon oxide can be formed during operation 108and/or other coating operations that involve, for example, CVD, ALD, orother deposition techniques.

Other operations in the process 100 that are specifically reflected inFIG. 1 may include compressing the electrode layer, furtherfunctionalizing the layer, and forming a battery electrode out of theelectrode layer. Additional downstream operations are explained below inthe context of electrode and battery arrangement description.

Electrode Layer Examples

The process described above can be used to prepare an electrode layer ofinterconnected hollow nanostructures containing high capacity activematerial. Hollow nanostructures can have a shape of hollow spheres,tubes, or other shapes, both regular and irregular. A largest innercross-sectional dimension of the cavities (e.g., an inner diameter forround structures) of the hollow nanostructures may be between about 1nanometer and 1000 nanometers or, more particularly, between about 5nanometers and 500 nanometers or even between about 10 nanometers and100 nanometers. The cavities provide free space for active materials toexpand into without causing substantial interference (e.g., pushing on)to other nanostructures or causing pulverization of the nanostructureitself. In specific embodiments, some or all of the hollownanostructures (e.g., at least about 50% of them) have cross-sectionaldimensions and/or sizes of openings into the hollow cavities that aretoo small for electrolyte to penetrate and fill the hollow cavities whenthe electrode layer is assembled into a battery. As such, the hollowcavities of the hollow nanostructures may remain substantially free fromelectrolyte, at least during normal battery operation.

In certain embodiments, the hollow nanostructures have a defined shellthickness. While other dimensions of the nanostructures may besufficiently large (e.g., several micrometer and even millimeters), theshell thickness is generally less than about 1000 nanometers. Asmentioned above, a thin shell may minimize pulverization of the highcapacity active materials during cycling. In certain embodiments, shellshave a thickness of between about 5 nanometers and 1000 nanometers onaverage or, more particularly, between about 10 nanometers and 500nanometers or even between about 50 nanometers and 250 nanometers. Aporosity of the electrode layer may be between about 20% and 90%including the hollow cavities of the nanostructures. In more specificembodiments, the porosity is between about 40% and 80%.

In certain embodiments, nanostructures have a substantially tubularshape. These nanotubes can be at least about 100 nanometers long or,more particularly, at least about 1 micrometer long or even at leastabout 50 micrometers long. The nanotubes may have joint structures alongtheir lengths. The joint structures provide physical or metallurgicalbonding between adjacent nanostructures. Generally, the joint structureprovides a direct electronic pathway between the adjacent structures,without requiring the presence of an external conductive agent (such ascarbon black) to provide a conductive pathway between the adjacentstructures.

An electrode layer may be bound to a conductive substrate. In certainembodiments, a conductive substrate has two electrode layers on eachside of the substrate. Various substrate examples are provided above.Electrode layers, which are sometimes referred to as active layers, canbe used as positive or negative electrodes in a battery as well as solidelectrolytes, which are further described below.

Rechargeable batteries assembled with these novel electrode layers canmaintain a stable electrochemical capacity of at least about 2000 mAh/gfor at least 100 cycles (based on the total weight of all activematerials present in the electrode layer). In the same or otherembodiments, batteries can maintain a stable electrochemical capacity ofat least about 700 mAh/g for at least about 100 cycles.

Electrode and Battery Arrangements

Electrode layer described above can be used to form positive and/ornegative battery electrodes. The battery electrodes are then typicallyassembled into a stack or a jelly roll. FIG. 5A illustrates a side viewof an aligned stack including a positive electrode 502, a negativeelectrode 504, and two sheets of the separator 506 a and 506 b inaccordance with certain embodiments. The positive electrode 502 may havea positive electrode layer 502 a and a positive uncoated substrateportion 502 b. Similarly, the negative electrode 504 may have a negativeelectrode layer 504 a and a negative uncoated substrate portion 504 b.In many embodiments, the exposed area of the negative electrode layer504 a is slightly larger that the exposed area of the positive electrodelayer 502 a to ensure trapping of the lithium ions released from thepositive electrode layer 502 a by insertion material of the negativeelectrode layer 504 a. In one embodiment, the negative electrode layer504 a extends at least between about 0.25 and 5 mm beyond the positiveelectrode layer 502 a in one or more directions (typically alldirections). In a more specific embodiment, the negative layer extendsbeyond the positive layer by between about 1 and 2 mm in one or moredirections. In certain embodiments, the edges of the separator sheets506 a and 506 b extend beyond the outer edges of at least the negativeelectrode layer 504 a to provide electronic insulation of the electrodefrom the other battery components. The positive uncoated portion 502 bmay be used for connecting to the positive terminal and may extendbeyond negative electrode 504 and/or the separator sheets 506 a and 506b. Likewise, the negative uncoated portion 504 b may be used forconnecting to the negative terminal and may extend beyond positiveelectrode 502 and/or the separator sheets 506 a and 506 b.

FIG. 5B illustrates a top view of the aligned stack. The positiveelectrode 502 is shown with two positive electrode layers 512 a and 512b on opposite sides of the flat positive current collector 502 b.Similarly, the negative electrode 504 is shown with two negativeelectrode layer 514 a and 514 b on opposite sides of the flat negativecurrent collector. Any gaps between the positive electrode layer 512 a,its corresponding separator sheet 506 a, and the corresponding negativeelectrode layer 514 a are usually minimal to non-existent, especiallyafter the first cycle of the cell. The electrodes and the separators areeither tightly would together in a jelly roll or are positioned in astack that is then inserted into a tight case. The electrodes and theseparator tend to swell inside the case after the electrolyte isintroduced and the first cycles remove any gaps or dry areas as lithiumions cycle the two electrodes and through the separator.

A wound design is a common arrangement. Long and narrow electrodes arewound together with two sheets of separator into a sub-assembly,sometimes referred to as a jellyroll, shaped and sized according to theinternal dimensions of a curved, often cylindrical, case. FIG. 6A showsa top view of a jelly roll comprising a positive electrode 606 and anegative electrode 604. The white spaces between the electrodesrepresent the separator sheets. The jelly roll is inserted into a case602. In some embodiments, the jellyroll may have a mandrel 608 insertedin the center that establishes an initial winding diameter and preventsthe inner winds from occupying the center axial region. The mandrel 608may be made of conductive material, and, in some embodiments, it may bea part of a cell terminal. FIG. 6B presents a perspective view of thejelly roll with a positive tab 612 and a negative tab 614 extending fromthe jelly roll. The tabs may be welded to the uncoated portions of theelectrode substrates.

The length and width of the electrodes depend on the overall dimensionsof the cell and thicknesses of electrode layers and current collector.For example, a conventional 18650 cell with 18 mm diameter and 65 mmlength may have electrodes that are between about 300 and 1000 mm long.Shorter electrodes corresponding to low rate/higher capacityapplications are thicker and have fewer winds.

A cylindrical design may be desirable for some lithium ion cells becausethe electrodes swell during cycling and exert pressure on the casing. Around casing may be made sufficiently thin and still maintain sufficientpressure. Prismatic cells may be similarly wound, but their case maybend along the longer sides from the internal pressure. Moreover, thepressure may not be even within different parts of the cells and thecorners of the prismatic cell may be left empty. Empty pockets may notbe desirable within the lithium ions cells because electrodes tend to beunevenly pushed into these pockets during electrode swelling. Moreover,the electrolyte may aggregate and leave dry areas between the electrodesin the pockets negative effecting lithium ion transport between theelectrodes. Nevertheless, for certain applications, such as thosedictated by rectangular form factors, prismatic cells are appropriate.In some embodiments, prismatic cells employ stacks rectangularelectrodes and separator sheets to avoid some of the difficultiesencountered with wound prismatic cells.

FIG. 7 illustrates a top view of a wound prismatic jellyroll. The jellyroll comprises a positive electrode 704 and a negative electrode 706.The white space between the electrodes is representative of theseparator sheets. The jelly roll is inserted into a rectangularprismatic case. Unlike cylindrical jellyrolls shown in FIGS. 6A and 6B,the winding of the prismatic jellyroll starts with a flat extendedsection in the middle of the jelly roll. In one embodiment, the jellyroll may include a mandrel (not shown) in the middle of the jellyrollonto which the electrodes and separator are wound.

FIG. 8A illustrates a side view of a stacked cell including a pluralityof sets (801 a, 801 b, and 801 c) of alternating positive and negativeelectrodes and a separator in between the electrodes. One advantage of astacked cell is that its stack can be made to almost any shape, and isparticularly suitable for prismatic cells. However, such cell typicallyrequires multiple sets of positive and negative electrodes and a morecomplicated alignment of the electrodes. The current collector tabstypically extend from each electrode and connected to an overall currentcollector leading to the cell terminal.

Once the electrodes are arranged as described above, the cell is filledwith electrolyte. The electrolyte in lithium ions cells may be liquid,solid, or gel. The lithium ion cells with the solid electrolyte alsoreferred to as a lithium polymer cells.

A typical liquid electrolyte comprises one or more solvents and one ormore salts, at least one of which includes lithium. During the firstcharge cycle (sometimes referred to as a formation cycle), the organicsolvent in the electrolyte can partially decompose on the negativeelectrode surface to form a solid electrolyte interphase layer (SEIlayer). The interphase is generally electrically insulating butionically conductive, allowing lithium ions to pass through. Theinterphase also prevents decomposition of the electrolyte in the latercharging sub-cycles.

Some examples of non-aqueous solvents suitable for some lithium ioncells include the following: cyclic carbonates (e.g., ethylene carbonate(EC), propylene carbonate (PC), butylene carbonate (BC) andvinylethylene carbonate (VEC)), vinylene carbonate (VC), lactones (e.g.,gamma-butyrolactone (GBL), gamma-valerolactone (GVL) and alpha-angelicalactone (AGL)), linear carbonates (e.g., dimethyl carbonate (DMC),methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl propylcarbonate (MPC), dipropyl carbonate (DPC), methyl butyl carbonate (NBC)and dibutyl carbonate (DBC)), ethers (e.g., tetrahydrofuran (THF),2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane (DME),1,2-diethoxyethane and 1,2-dibutoxyethane), nitrites (e.g., acetonitrileand adiponitrile) linear esters (e.g., methyl propionate, methylpivalate, butyl pivalate and octyl pivalate), amides (e.g., dimethylformamide), organic phosphates (e.g., trimethyl phosphate and trioctylphosphate), and organic compounds containing an S═O group (e.g.,dimethyl sulfone and divinyl sulfone), and combinations thereof.

Non-aqueous liquid solvents can be employed in combination. Examples ofthe combinations include combinations of cyclic carbonate-linearcarbonate, cyclic carbonate-lactone, cyclic carbonate-lactone-linearcarbonate, cyclic carbonate-linear carbonate-lactone, cycliccarbonate-linear carbonate-ether, and cyclic carbonate-linearcarbonate-linear ester. In one embodiment, a cyclic carbonate may becombined with a linear ester. Moreover, a cyclic carbonate may becombined with a lactone and a linear ester. In a specific embodiment,the ratio of a cyclic carbonate to a linear ester is between about 1:9to 10:0, preferably 2:8 to 7:3, by volume.

A salt for liquid electrolytes may include one or more of the following:LiPF₆, LiBF₄, LiClO₄ LiAsF₆, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiCF₃SO₃,LiC(CF₃SO₂)₃, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃, LiPF₃(iso-C₃F₇)₃,LiPF₅(iso-C₃F₇), lithium salts having cyclic alkyl groups (e.g.,(CF₂)₂(SO₂)_(2x)Li and (CF₂)₃(SO₂)_(2x)Li), and combination of thereof.Common combinations include LiPF₆ and LiBF₄, LiPF₆ and LiN(CF₃SO₂)₂,LiBF₄ and LiN(CF₃SO₂)₂.

In one embodiment the total concentration of salt in a liquid nonaqueoussolvent (or combination of solvents) is at least about 0.3 M; in a morespecific embodiment, the salt concentration is at least about 0.7M. Theupper concentration limit may be driven by a solubility limit or may beno greater than about 2.5 M: in a more specific embodiment, no more thanabout 1.5 M.

A solid electrolyte is typically used without the separator because itserves as the separator itself. It is electrically insulating, ionicallyconductive, and electrochemically stable. In the solid electrolyteconfiguration, a lithium containing salt, which could be the same as forthe liquid electrolyte cells described above, is employed but ratherthan being dissolved in an organic solvent, it is held in a solidpolymer composite. Examples of solid polymer electrolytes may beionically conductive polymers prepared from monomers containing atomshaving lone pairs of electrons available for the lithium ions ofelectrolyte salts to attach to and move between during conduction, suchas Polyvinylidene fluoride (PVDF) or chloride or copolymer of theirderivatives, Poly(chlorotrifluoroethylene),poly(ethylene-chlorotrifluoro-ethylene), or poly(fluorinatedethylene-propylene), Polyethylene oxide (PEO) and oxymethylene linkedPEO, PEO-PPO-PEO crosslinked with trifunctional urethane,Poly(bis(methoxy-ethoxy-ethoxide))-phosphazene (MEEP), Triol-type PEOcrosslinked with difunctional urethane,Poly((oligo)oxyethylene)methacrylate-co-alkali metal methacrylate,Polyacrylonitrile (PAN), Polymethylmethacrylate (PNMA),Polymethylacrylonitrile (PMAN), Polysiloxanes and their copolymers andderivatives, Acrylate-based polymer, other similar solvent-freepolymers, combinations of the foregoing polymers either condensed orcross-linked to form a different polymer, and physical mixtures of anyof the foregoing polymers. Other less conductive polymers may be used incombination with the above polymers to improve strength of thinlaminates include: polyester (PET), polypropylene (PP), polyethylenenapthalate (PEN), polyvinylidene fluoride (PVDF), polycarbonate (PC),polyphenylene sulfide (PPS), and polytetrafluoroethylene (PTFE).

FIG. 9 illustrates a cross-section view of the wound cylindrical cell inaccordance with one embodiment. A jelly roll comprises a spirally woundpositive electrode 902, a negative electrode 904, and two sheets of theseparator 906. The jelly roll is inserted into a cell case 916, and acap 918 and gasket 920 are used to seal the cell. It should be note thatin certain embodiments a cell is not sealed until after subsequentoperations (i.e., operation 208). In some cases, cap 912 or case 916includes a safety device. For example, a safety vent or burst valve maybe employed to break open if excessive pressure builds up in thebattery. In certain embodiments, a one-way gas release valve is includedto release oxygen released during activation of the positive material.Also, a positive thermal coefficient (PTC) device may be incorporatedinto the conductive pathway of cap 918 to reduce the damage that mightresult if the cell suffered a short circuit. The external surface of thecap 918 may used as the positive terminal, while the external surface ofthe cell case 916 may serve as the negative terminal. In an alternativeembodiment, the polarity of the battery is reversed and the externalsurface of the cap 918 is used as the negative terminal, while theexternal surface of the cell case 916 serves as the positive terminal.Tabs 908 and 910 may be used to establish a connection between thepositive and negative electrodes and the corresponding terminals.Appropriate insulating gaskets 914 and 912 may be inserted to preventthe possibility of internal shorting. For example, a Kapton™ film mayused for internal insulation. During fabrication, the cap 918 may becrimped to the case 916 in order to seal the cell. However prior to thisoperation, electrolyte (not shown) is added to fill the porous spaces ofthe jelly roll.

A rigid case is typically required for lithium ion cells, while lithiumpolymer cells may be packed into a flexible, foil-type (polymerlaminate) case. A variety of materials can be chosen for the case. Forlithium-ion batteries, Ti-6-4, other Ti alloys, Al, Al alloys, and 300series stainless steels may be suitable for the positive conductive caseportions and end caps, and commercially pure Ti, Ti alloys, Cu, Al, Alalloys, Ni, Pb, and stainless steels may be suitable for the negativeconductive case portions and end caps.

In addition to the battery applications described above, metal silicidesmay be used in fuel cells (e.g., for negative electrodes, positiveelectrodes, and electrolytes), hetero-junction solar cell activematerials, various forms of current collectors, and/or absorptioncoatings. Some of these applications can benefit from a high surfacearea provided by metal silicide structures, high conductivity ofsilicide materials, and fast inexpensive deposition techniques.

CONCLUSION

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing the processes, systems and apparatus of the presentinvention. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein.

1.-11. (canceled)
 12. An electrode layer for use in a rechargeablebattery, the electrode layer comprising: a plurality of interconnectedhollow tubular nanostructures having shells around internal cavities,wherein the shells comprise high capacity electrochemically activematerial, wherein the internal cavities provide free space for the highcapacity active material to swell into during cycling of therechargeable battery.
 13. The electrode layer of claim 12, wherein theinternal cavities are substantially inaccessible to an electrolyte ofthe rechargeable battery.
 14. The electrode layer of claim 12, whereinthe electrode layer is configured to provide a stable energy capacity ofat least about 2000 mAh/g after at least 100 cycles based on the weightof the high capacity electrochemically active material.
 15. Theelectrode layer of claim 12, further comprising junction structuresinterconnecting two or more interconnected hollow nanostructures, saidjunction structures comprise one or more materials selected from thegroup consisting of the high capacity electrochemically active material,metal, and a polymeric binder.
 16. The electrode layer of claim 15,further comprising a conductive substrate, wherein additional junctionstructures interconnect one or more interconnected hollow nanostructuresand said conductive substrate.
 17. The electrode layer of claim 12,wherein the high capacity electrochemically active material comprisesone or more materials selected from the group consisting of silicon,tin, and germanium.
 18. The electrode layer of claim 12, furthercomprising an outer layer substantially covering an outer surface of theinterconnected hollow nanostructures, wherein the outer layer comprisingone or more material selected from the group consisting of carbon,titanium, silicon, aluminum, and copper.
 19. The electrode layer ofclaim 12, wherein the electrode layer has a porosity of between about20% and 80%.
 20. The electrode layer of claim 12, wherein therechargeable battery is a lithium ion battery and the high capacityelectrochemically active material is a negative active material.
 21. Alithium ion battery comprising: an electrode layer comprising: aplurality of interconnected hollow tubular nanostructures having shellsaround internal cavities, wherein the shells comprise high capacityelectrochemically active material, wherein the internal cavities providefree space for the high capacity active material to swell into duringcycling of the rechargeable battery.
 22. An electrode layer for use in arechargeable battery, the electrode layer comprising: interconnectedhollow nanostructures having shells around internal cavities, whereinthe shells comprise a high capacity electrochemically active material;and a substrate underlying the interconnected hollow nanostructures,wherein at least some of the interconnected hollow nanostructures areinterconnected at points above the substrate by first structures, thefirst structures including one or more of metals and high capacityelectrochemically active material and wherein the internal cavitiesprovide free space for the high capacity active material to swell intoduring cycling of the rechargeable battery.
 23. The electrode layer ofclaim 22, wherein the internal cavities are substantially inaccessibleto an electrolyte of the rechargeable battery.
 24. The electrode layerof claim 22, wherein the electrode layer is configured to provide astable energy capacity of at least about 2000 mAh/g after at least 100cycles based on the weight of the high capacity electrochemically activematerial.
 25. The electrode layer of claim 22, wherein the high capacityelectrochemically active material comprises one or more materialsselected from the group consisting of silicon, tin, and germanium. 26.The electrode layer of claim 22, further comprising an outer layersubstantially covering an outer surface of the interconnected hollownanostructures, wherein the outer layer comprising one or more materialselected from the group consisting of carbon, titanium, silicon,aluminum, and copper.
 27. The electrode layer of claim 22, wherein theelectrode layer has a porosity of between about 20% and 80%.
 28. Theelectrode layer of claim 22, wherein the rechargeable battery is alithium ion battery and the high capacity electrochemically activematerial is a negative active material.
 29. The electrode layer of claim12, further comprising an outer layer substantially covering an outersurface of the interconnected hollow nanostructures, wherein the outerlayer comprises one or more materials selected from the group consistingof carbon, a metal, a metal oxide, a metal nitride, and a metal carbide.30. The electrode layer of claim 12, wherein the hollow nanostructureshave an aspect ratio of at least about four and a length of at leastabout 5 micrometers.
 31. The electrode layer of claim 12, wherein thehollow nanostructures have a largest inner cross-sectional dimensionbetween about 5 nanometers and 500 nanometers.
 32. The electrode layerof claim 12, wherein the shells have a thickness between about 5 and1000 nm.